This invention relates to a magnetic resonance(MR) imaging system and more particularly to an MR imaging system for acquiring image data in a rapid scan.
MR imaging is a remarkable technique for a medical diagnosis because a good contrast image for head or spine tissue can be acquired.
An MR imaging system for acquiring image data in a few seconds can suppress an artifact caused by respiratory motion.
Furthermore, there is known a possibility of acquiring image data in a fraction of a second, with echo planar imaging(EPI).
The MR imaging system using EPI operates as follows. First, a 90.degree. pulse is applied; and following the 90.degree. pulse, 180.degree. pulse a is applied. Then gradients are superimposed on a static magnetic field dephase spins. A gradient of reverse polarity is then applied in a predetermined direction to refocus the spins, and a first echo is acquired. The phase encoding gradient is then immediately incremented and the same procedure is repeated and second echo is acquired. In this way, all data in a k-space can be sampled in one shot.
Furthermore, with this technique, motion such as in a heart, elbow or knee of a human body, can be viewed in essentially real time.
However, in order to acquire data in a fraction of a second with EPI, very large gradient amplitudes are required, and large gradient amplitudes require large gradient coil currents. For example, a 5 mT/m gradient requires about 500 A current in a gradient coil in which a human body can fit.
Further, rapid gradient switching, for example at 100 .mu. second, is required.
In a conventional MR imaging system with EPI, a very high voltage gradient coil is required which in turn requires a very high capability-power source such a gradient power source requires extra space for installation and extra cost, and uses a great amount of electric energy.
These problems also occur with other imaging techniques, for example, techniques for imaging blood flow or angiography which also require very large gradient amplitudes, rapid switching capabilities and higher sampling rate.
The Japanese Patent Laid-Open Publication No. 64-20835. filed Jul. 16, 1987, and published Jan. 24, 1989, discloses an MR imaging system comprising a gradient coil for superimposing a gradient in three mutually orthgonal axes on a static magnetic field and comprises an inversion gradient coil for applying an alternating gradient, wherein said inversion gradient coil is provided independently from said gradient coil.
U.S. Pat. No. 4,926,125, issued May 15, 1990, discloses a surface gradient assembly for high-speed, high-resolution NMR imaging comprising at least one substantially planar gradient coil.